How can I swap Textures and Materials
On replacing Textures

You don't actually have to delete the texture. Instead you can "reassign" the slot by just opening a new one. 

Assuming you have already a material with a texture


Go to Properties Window -> Texture Panel
Select the desired Texture slot
Scroll down to "Image" 


Now here are two possibilities: 

If you want to switch between two textures to chose which one looks better, hit the upper folder button and open a new image texture. 



Notice how you now have two textures in your selection box?
If you want to switch between them, just click the selection box and change the texture. 



If however you want to replace the texture with another one, hit the lower folder button (under Source:)



Notice how your original texture got replaced with the new one

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On replacing Materials


Go to Properties Window -> Material Panel
Hit the "+" Button on the right side of the Material Selector. This basically creates a new material slot.
Select the new slot and hit "New".
Make changes to your new Material (Textures, colors, whatever) 
With that new material selected Tab into Edit mode and hit "Assign". 




You still have both materials to chose from.